This invention relates to fluid-permeable agents for non-woven sheets made of polyolefin fibers such as composite synthetic fibers having sheath-core structure made of two or more polymers with different melting points having polyolefin polymer sheaths and also to application methods of such agents. More particularly, this invention relates to agents to be applied to such fibers for providing durability and fluid-permeability and also to methods of applying such agents.
Recently, non-woven sheets by a dry-bonding process and more particularly bondable non-woven sheets are coming to be frequently used in medical supplies and hygienical articles. For diapers, napkins and the like, polyolefin fibers and composite polyethylene fibers are frequently used in view of their skin-comfortability (that is, softness and absence of discomfort from wetting). In order to improve product characteristics such as bulkiness, restorability and shape-stability against heat, on the other hand, use is frequently made of heat-bondable composite fibers comprised of polyester fibers and polypropylene fibers as the core and polyolefin polymers as the sheath.
In order to eliminate user's discomfort from a diaper, a napkin and the like caused by sweat, urine, menstrual fluid and other body fluids, it is considered important not only that the body-facing parts of these products be wettable but also their wettability can be quickly manifested. For this reason, fluid-permeability within a short time is a required characteristic of polyolefin fibers of which these body-facing parts are comprised. Since diapers, in particular, are generally worn by infants, seniors and very sick persons who cannot take care of themselves, a single diaper should be able to handle two or more discharges without causing discomfort to the wearer. For this reason, durability of fluid-permeability (or durability against repeated use) is another strongly required characteristic.
Prior art methods of providing fluid-permeability to polyolefin fibers and sheaths of polyolefin composite fibers include (1) application of a low-molecular weight hydrophilic compound, (U.S. Pat. Nos. 3664,343 and 3,821,021 and U.S. patent application Ser. No. 210,636 filed Jun. 23, 1988, now abandoned assigned to the present assignee) (2) application of a hydrophilic macromolecular resin, (U.S. Pat. Nos. 3,934,587, 4,297,410, 4,406,660 and 4,718,899) (3) improvement of surface characteristics by chemical processing, solvent processing, plasma processing, corona discharge processing, etc. By the first of the above methods, however, not only desired fluid-permeability cannot be obtained because these agents cannot wet the fiber surface satisfactorily but also there is no durability even if fluid-permeability can be obtained to a certain extent and, in many instances, the user's skin is seriously irritated. The second of the above methods, on the other hand, generally cannot provide sufficient fluid-permeability. Additional disadvantages include insufficiency in durability if use is made of an agent which can provide fluid-permeability to a certain extent. Moreover, agents of this type have the tendency of causing troubles of various kinds during the production process of non-woven sheets. As for the third of the above methods, although it provides relatively favorable results regarding skin-irritation and permeability, resultant fluid-permeability tends to deteriorate with time along with the polar groups which are generated by the surface changes of the fibers. In other words, this method not only provides insufficient durability but also is itself uneconomical.
There have been proposals, on the other hand, to use a hydrophilic polymer as base material and to partially coat its surface with a hydrophobic compound. A water repellant polymer may be used as the hydrophobic compound as disclosed in U.S. Pat. No. 3,934,587. Alternatively, us may be made of a compound of silicon or fluorine as disclosed in U.S. Pat. No. 3,838,692. These proposed methods are equally unsatisfactory because a basically hydrophilic polymer is used as the base material and the aforementioned characteristics of basically hydrophobic polyolefin fibers are lacking.